1. Chapter 04: Modern Applications
CS 408 Computer Networks
Chapter 04: Modern Applications

2. Hypertext Transfer Protocol HTTP
What does hypertext mean?
“a body of written or pictorial material interconnected in such a complex way that it could not conveniently be presented or represented on paper”
Ted Nelson, 1965
Underlying protocol of the World Wide Web
Can transfer plain text, audio, images, etc.
actually you can transfer any type of file using HTTP
Most recent version HTTP 1.1 – RFC 2616
176 pages

3. HTTP Overview Transaction oriented client/server protocol
Usually between Web browser (client) and Web server
Uses TCP connections (on port 80)
Stateless
Server (normally) does not keep any info about client history
Each transaction treated independently
New TCP connection for each transaction
Terminate connection when transaction is complete
That does not mean that, say, 20 new connections are needed to download 20 different items from a web site.
It is possible to have “persistent” connections that several items are downloaded back-to-back
Why stateless?
any idea?
Hint: it was a design decision due to the nature of transactions

4. Examples of HTTP Operation
end-to-end direct connection
intermediate nodes such as proxy
use of cache

5. HTTP Messages Simple request/response mechanism Request Response
Client to server
Response
Server to client
First, client opens a TCP connection towards the server at port 80.

6. HTTP Message Structure
Response(status) Line /

7. Request
Request-Line
Method <SP> Request_URL <SP> HTTP/Version <CRLF>
Several Methods - some examples (see the book for the full list)
Get
Head
Delete
Put
Example
GET /index.html HTTP/1.1

8. General Header Fields
Contain information that is not directly related to data to be transferred
but mostly directives to intermediate nodes
some are for connection management
for example
Keep-alive: to keep the TCP connection open for a while; needed for persistent connections (shall see persistent connections later)
can be used for both request and response

9. Request Header Field
Additional parameters about requests - some examples (see the book for the full list)
Accept charset
Accept language
Host
If modified since
can be used with GET command
Referrer

10. Response Messages
Status line followed by one or more general, response and entity headers, followed by entity body
Status-Line
HTTP-Version <SP> Status-Code <SP> Reason-Phrase
some examples for “status-code – reason-phrase” pairs (see the book for the full list)
200 OK
404 Not found
405 Method not allowed
400 Bad request

11. Response Header Fields
Additional info about the response
Some examples (see the book for the full list)
Location: exact location of the requested URL
Server: info about server software

12. Entity Header Information about the entity to be sent by the server
similar to MIME format
Some examples (see the book for the full list)
Content language
Content length
Content type
Last modified
etc.

13. Entity Body
Arbitrary sequence of octets that constitutes the transferred entity (actual data)
HTTP transfers any type of data including:
text
binary data
audio
images
video
Interpretation of data determined by header fields

14. HTTP request message The rest of HTTP discussion is from Kurose&Ross
ASCII (human-readable format)
Example:
request line
(GET, PUT,
HEAD, etc. commands)
GET /somedir/page.html HTTP/1.1
Connection: close
Host:
User-agent: Mozilla/4.0
Accept-language:fr
(extra carriage return, line feed)
header
lines
Carriage return,
line feed
indicates end
of message
First open a TCP connection (you may use telnet for this) to the host at port 80

15. HTTP response message (example)
status line
(protocol
status code
status phrase)
HTTP/ OK
Connection close
Date: Thu, 06 Aug :00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data data data data data ...
header
lines
data, e.g.,
requested
HTML file

16. HTTP connections Nonpersistent HTTP
Only one object is sent over a TCP connection.
HTTP/1.0 used only nonpersistent HTTP
Persistent HTTP
Multiple objects can be sent over single TCP connection between client and server.
HTTP/1.1 uses both persistent and nonpersistent connections

17. Nonpersistent HTTP
Suppose user enters URL
(contains text,
references to 10
jpeg images)
1. HTTP client initiates TCP connection to HTTP server (process) at on port 80
2. HTTP server at host waiting for TCP connection at port 80. “accepts” connection and notifies client
3. HTTP client sends HTTP request message into TCP connection socket. Message indicates that client wants object /someDepartment/home.index
time
4. HTTP server receives request message, forms response message containing requested object, and sends message into its socket. After that, server closes TCP connection
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
6. Steps 1-5 repeated for each of 10 jpeg objects

18. Response time modeling
time to
transmit
file
initiate TCP
connection
RTT
request
received
time
Definition of RRT (round trip time): time needed for a small packet to travel from client to server and back (basically 2*prop. delay).
Response time:
one RTT to initiate TCP connection
one RTT for HTTP request and first few bytes of HTTP response to return
file transmission time
total = 2RTT + file transmission time

19. Persistent HTTP Nonpersistent HTTP issues: Persistent HTTP
requires 2 RTTs per object (plus the transmission time)
but browsers often open parallel TCP connections to fetch referenced objects
Client and server should allocate resources for each TCP connection
Persistent HTTP
server leaves TCP connection open after sending response
subsequent HTTP messages between same client/server are sent over this connection

20. Pipelining in Persistent HTTP
Persistent without pipelining:
client issues new request only when previous response has been received
one RTT for each referenced object (plus the transmission time)
Another RTT is needed for TCP connection, but this is only once for the entire connection
Persistent with pipelining:
default in HTTP/1.1
client sends requests as soon as it encounters a referenced object
as little as one RTT for all the referenced objects (plus the transmission times)
Another RTT plus the transmission time may be needed for the main object where the references are learnt

21. Cookies: keeping “state”
Many major Web sites use cookies to remember their clients
Four components:
1) cookie header line in the HTTP response message
2) cookie header line in HTTP request message
3) cookie file kept on user’s host and managed by user’s browser
4) back-end database at Web site
Example:
- Susan access Internet always from same PC
- She visits a specific e-commerce site for first time
- When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database using this ID
- One week later, when Susan visits the same site, the site remembers her
this part is adapted from Kurose&Ross, Computer Networking

22. Cookies: keeping “state” (cont.)
client
Server (amazon)
Cookie file
ebay: 8734
usual http request msg
server
creates ID
1678 for user
usual http response +
Set-cookie: 1678
entry in backend
database
Cookie file
amazon: 1678
ebay: 8734
one week later:
access
usual http request msg
cookie: 1678
Cookie file
amazon: 1678
ebay: 8734
cookie-
specific
action
usual http response msg

23. Cookies (continued) What cookies can bring: Cookies and privacy:
Identification
User session state (server remembers where client stopped last time)
Customization
Shopping carts
Cookies and privacy:
cookies allow sites to learn a lot about you
and may sell this info
advertising companies obtain info across sites about your browsing pattern using banner ads that contain cookies

24. Internet Directory Services DNS
Domain Name System
a directory lookup service
Provides mapping between host name and IP address
A “must” for proper to functioning of Internet
RFCs 1034 (concepts) and 1035 (implementation)
1987
total 110 pages
Updated by many other RFCs

25. Internet Directory Services DNS
Four important elements of DNS
Domain name space
Tree-structured
DNS database (distributed)
The info about each node in name space tree structure is contained in a Resource Record (RR).
The collection of RRs is organized as a distributed database
Name servers
Servers that hold and process information about portion of tree and corresponding RRs
Name Resolvers
Programs that help clients to extract information from name servers

26. Domain Names 32-bit IPv4 addresses uniquely identify devices
Network number, Host address, later subnet addresses
Routers route based on network numbers
People tend to memorize names, not numbers
a naming mechanism is needed
In Arpanet times, hosts.txt file was used
managed centrally, downloaded by all hosts daily
become insufficient in time
In the Internet, naming problem is addressed by the concept of domain
Group of hosts that have common naming elements
.com domain, .edu.tr domain, sabanciuniv.edu domain
Organized hierarchically
Names are assigned to reflect hierarchical organization
.tr .edu.tr .boun.edu.tr

27. Portion of Internet Domain Tree
Top level domains
Labels at most 63 chars, Full name at most 255 chars
Case insensitive
over 200 TLDs (including later added ones, e.g. .biz .pro .info)
hierarchy helps uniqueness (explain this in CS terms!)
Do you know the char length limits?
Naming follows organizational boundaries, not physical ones

28. Domain Names and Example
Variable-depth unlimited levels hierarchy for names (labels)
Delimited by period (.)
edu is college-level educational institutions
yale.edu is domain for Yale University in US
should yale.edu have an IP address?
not necessary, but it has ( )
cs.yale.edu is Computer Science department at Yale
has an IP address ( )
Eventually get to leaf nodes
Identify specific hosts
Hosts are assigned Internet (IP) addresses

29. DNS Database
Each TLD and subordinate nodes manage uniqueness of the names that they assign
Management of subordinate domains may be delegated
down the hierarchy
In this way, zones are created
Distributed database
Thousands of zones
each of these zones are separately managed by different name servers

30. Zones Each non-leaf node may or may not manage its childs
cs.yale.edu would like to run its own name server, but eng.yale.edu not
Next: How can we represent a zone in the database?
but before, we have to understand the structure of resource records

31. Resource Record - 1
Records in a DNS database are called Resource Records (RRs)
info about hosts
there are different types of RRs
Fields of one RR
Name TTL Class Type Value
Domain name
Series of labels of alphanumeric characters or hyphens
Labels are separated by period (“.”)
Type
of the RR. We will see now

32. Resource Record - 2 RR Fields (cont’d) Class Time to live (TTL)
Potentially DNS can be used for naming in several other systems
Usually IN, for Internet
Time to live (TTL)
How long to hold the result in local cache
Zero means don’t cache
Value (Rdata)
Resource data
For each RR type interpretation is different
For A type, Rdata is 32-bit IP address

33. Resource Record Types - 1A
Address type. Value of A type RRs is an IP address
SOA
Start of Authority
Parameters (mostly to sync with other servers) and info about this zone MX
Mail Exchange
Value field is the name of the receiving SMTP agent for the Domain_Name
may be more than one MX RRs for one domain
Mostly for load balancing for the domains that receive high volume of s
Multiple MX RRs: DNS Server sends out full list list of MX RRs but in different order in every query.

34. Resource Record Types - 2
CNAME
Canonical Name
used to create aliases
Value field is the canonical host name (for the alias, which is given as Domain_Name) NS
Name Server
Value field is the name of the server who knows the IP addresses of the hosts that belong to the domain given in the Domain_Name field.
can be used to specify the names of the name servers in both current domain or in subordinate domains (for delegation purposes)
There might be several DNS servers for each domain for fault tolerance

35. Resource Record Types - 3
PTR
Pointer type
used for reverse lookups
Domain_Name field is an IP address (but in a differently formatted way); Value is the hostname
HINFO
Host Info.
OS and processor type of information about the zone’s server and hosts
TXT
Textual comments

36. A portion of a possible DNS database for cs.vu.nl.
cs.vu.nl IN NS flits.cs.vu.nl.
cs.vu.nl IN NS star.cs.vu.nl.
zephyr.cs.vu.nl IN A
zephyr.cs.vu.nl IN HINFO Sun Unix
star.cs.vu.nl IN A
star.cs.vu.nl IN A
star.cs.vu.nl IN HINFO Sun Unix

37. Addition to previous example
How to delegate a subzone ai.cs.vu.nl?
Add the following RRs to database for cs.vu.nl
ai.cs.vu.nl IN NS dns.ai.cs.vu.nl.
dns.ai.cs.vu.nl IN A ;IP address of dns.ai.cs.vu.nl
These two RRs are together called “glue record”

38. A Better Example of SOA RR
anynet.com IN SOA dns.anynet.com. admin.anynet.com ( ; Serial
; Refresh
; Retry
; Expire
86400) ; Minimum )
Admin’s address
Serial number: DNS admin changes this value after each update. Generally it reflects update date.
The refresh interval in the REFRESH field is the length of time that a secondary server waits after successful replication of the database before it attempts replication again.
The retry interval in the RETRY field is the length of time that a secondary server waits after an unsuccessful replication attempt before it attempts replication again.
The expiry interval in the EXPIRE field is the length of time that a secondary server holds on to the old data that it already has whilst its attempts at replication remain unsuccessful.
Minimum: Default TTL of the zone. This is default value for RRs which do not specify a TTL in its own record and for nonexisting domains (negative caching).
Host name of the primary name server of the zone

39. The mystery behind different IPs for the same host
For load balancing
Works in round-robin fashion
albertlevi.com IN A
albertlevi.com IN A
albertlevi.com IN A
First query returns , second query returns , third returns , forth , ...
Or one query returns all IP addresses, but in different order in every other query

40. Example for PTR record for Reverse Lookup
Useful when you know the IP address and want to know the corresponding host name
Suppose you would like to know the host name for IP address
you have to query the DNS servers for the PTR entry
in-addr.arpa.
Be careful! numbers are in reverse order
In order to find the host name, the host’s name server should have an entry
in-addr.arpa. PTR domain_name
for this particular case domain_name is uveyik.cc.boun.edu.tr

41. Reverse DNS for 193.140.192.24 (was) Generated by www.DNSstuff.com
Preparation:
The reverse DNS entry for an IP is found by reversing the IP, adding it to "in-addr.arpa", and looking up the PTR record.
So, the reverse DNS entry for is found by looking up the PTR record for in-addr.arpa.
All DNS requests start by asking the root servers, and they let us know what to do next.
How I am searching:
Asking e.root-servers.net for in-addr.arpa PTR record:
e.root-servers.net says to go to sec3.apnic.net. (zone: 193.in-addr.arpa.)
Asking sec3.apnic.net. for in-addr.arpa PTR record:
sec3.apnic.net [ ] says to go to ns1.ulakbim.gov.tr. (zone: in-addr.arpa.)
Asking ns1.ulakbim.gov.tr. for in-addr.arpa PTR record:
ns1.ulakbim.gov.tr [ ] says to go to asiyan.cc.boun.edu.tr. (zone: in-addr.arpa.)
Asking asiyan.cc.boun.edu.tr. for in-addr.arpa PTR record: Reports kennedy.cc.boun.edu.tr. [from ]
Answer:
PTR record: kennedy.cc.boun.edu.tr. [TTL 3600s] [A= ]
Try mxtoolbox.com or for online DNS lookup or use nslookup command

42. Typical DNS Operation
User program requests IP address for a domain name
Resolver module in local host formulates query for local name server
In same domain as resolver
Local name server checks for name in local database and cache
If so, returns IP address to requestor
Otherwise, query other available name servers
Starting down from root of DNS tree
Local name server caches the reply
and maintain it for TTL seconds
User program is given IP address or error message

43. DNS Name Resolution
local

44. Root Name Servers servers for TLDs
local server starts with a root server if it does not know anything about the domain to be resolved
actually there are several of them worldwide
listed in configuration files of the name servers
Figure from Kurose-Ross

45. Authoritative Name Servers
A relative concept
the authoritative name server of a host is the one that keeps the A type RR of that host
Actually a local name server is also authoritative name server for all of the hosts in its zone
In principle, DNS queries aim to reach the authoritative name server for the host to be resolved
but generally responses come from the other servers that already cached the requested record
that is why the nslookup responses are mostly non-authoritative
DNS name servers automatically send out updates to other relevant name servers for quick response
mechanisms designed in RFC 2136 and not in the scope of CS408
nslookup is an application level command line tool for DNS inquiry. It is available in most operating systems (just write “nslookup” at the command line to try).
nslookup is an application level command line tool for DNS inquiry. It is available in most operating systems (just write “nslookup” at the command line to try).

46. Iterative vs. Recursive Queries
If one name server does not know the queried host, it acts like a DNS client and asks to next name server in the zone hierarchy.
Then sends the result back recursively
Iterative
If the name server does not know the host, then returns the address of the next server in the zone hierarchy, but does not ask that server.
The name servers learns about the next one in the hierarchy using the glue records.
Remark: Queries and responses are sent over UDP (mostly)
Why?

47. Example - 1 looking for the IP address of gaia.cs.umass.edu
Recursive queries
Let’s think about cached alternatives

48. Example - 2 looking for the IP address of gaia.cs.umass.edu
Recursive and iterative queries

49. DNS Message Format

50. DNS Message Fields - Header
Header always present 
Identifier to match queries and responses.
Query / Response: is message query or response?
Opcode: Standard or inverse query (address to name), or server status request
Authoritative Answer: is the response authoritative?
Truncated: was response truncated
Requestor will use TCP to resend query
Recursion Desired
Recursion Available
Response Code: e.g. no error, format error, name does not exist
QDcount: # of entries in question section (zero or more)
ANcount: # of RRs in answer section (zero or more)
NScount: # of RRs in authority section (zero or more)
ARcount: # of RRs in additional records section (zero or more)

51. DNS Message Fields – Question and Answers
Domain Name
Sequence of labels for the domain name to be resolved
Each label has its length field beforehand
Query Type
what type of RR is requested?
Query Class: typically Internet.
Answer section contains RRs that answer question
Authority section contains RRs that point toward an authoritative name server

52. Sockets
covered in labs